IPNs have been known for more than 40 years. IPNs are defined as macromolecular assemblies comprising two or more polymers wherein at least one is in the form of a network, the polymers are at least partially interlaced on a molecular scale but not covalently bonded to each other.
Because there is no chemical bond between the networks (or polymer/network), each network may retain its individual properties independently of its individual proportion in the blend. As a result an improvement can be attained in properties such as mechanical strength, impact resistance, and toughness and other. There are two main types of IPNs, viz semi-IPN where at least one component is not in network form, and full IPN where all species are in network form. The term “IPN” as used herein comprises both semi-IPNs and full IPNs.
US 2002/01222946 relates to a process of producing an IPN comprising: providing a liquid mixture comprising silicone oligomers and silsesquioxane oligomers; and curing the liquid mixture to form a composition of first and second polymers, the first polymer comprising the silsesquioxane oligomers cross-linked by siloxane bonds and the second polymer comprising a cross-linked silicone network formed in part from the silicone oligomers.
US 20030000028 provides a colorant for use in tinting contact lenses in which the binding polymer used is capable of forming an interpenetrating polymer network with the lens material. When the colorants of the invention are applied to uncured lens material that is subsequently cured, the binding polymer forms an interpenetrating polymer network with the lens material embedding the colorant within the lens material, resulting in a stable, tinted lens.
Several other methods for producing IPNs are known. The methods e.g. include simultaneously forming and cross-linking the polymer networks in the presence of each other. Other methods include forming a first network and thereafter swelling this network with monomers, cross-linking agents and optionally initiator/catalyst with or without solvents where after this monomer may form a polymer and optionally a network, e.g. as disclosed in US 2002/0052448.
Applicants co-pending US 2006/0148985 relates to a method of producing an IPN by forming a first polymer substrate and thereafter swelling this substrate with monomers in the presence of CO2 in supercritical or liquid state. The monomers may be induced to form a polymer and optionally cross-linked to form a network.